The present invention relates to a device for a balanced to unbalanced line transformation (balun).
A balun is a device which is used as a transition between an unbalanced line and a balanced line. An unbalanced two-wire line is one having one of the two conductors grounded, such as a coaxial cable with the outer shield grounded. A balanced two-wire line is one in which neither of the two conductors is grounded and both have essentially identical impedances to ground. A transmission line may also be partially unbalanced if one wire has a lower impedance to ground than the other wire. A balun is usually a transformer which allows electrical isolation of the unbalanced line from the balanced line. The use of a transformer also allows impedance matching from the unbalanced line to the balanced line. This is done by varying the ratio of the number of turns of the balanced and unbalanced wires on the transformer. A bifilar winding is often used to improve the coupling between the primary and secondary of the transformer. A bifilar winding is the winding in close proximity of the primary and secondary wires over the same core.
At high frequencies, a transformer's efficiency as a balun is limited because of limits in the value of the coefficient of coupling of a transformer. A transformer can be connected as a choke, with there being a direct electrical connection from input to output, rather than electrically isolating the input and the output with a standard transformer connection. A balun which is connected as a choke will perform the balun function of isolating the balanced and the unbalanced lines if the open circuit inductance is sufficiently large at the desired frequencies compared to the impedance to ground from the balanced lines.
The desired signal which is transmitted along the two wire conductors and through the balun is a differential signal (i.e., the signal appears between the two wires of the line). Electrical noise will generate an interfering signal which is picked up equally by both conductors and is called a common mode signal (i.e., the signal appearing between each of the conductors and ground). The standard choke-type balun does not substantially attenuate the common mode interference signal which may be picked up by either the balanced or unbalanced line.
In certain applications, such as the transmission of computer data, there is a need for a broad frequency band balun, with high common mode rejection. The digital data transmitted may go as low as DC and hiqher than the operating bit rate which presently can be as great as 10 MHz. Neither a standard transformer-type balun nor a standard choke-type balun is suitable for this application because of the problems referred to above. In addition, in order to keep costs down it is desirable to use low cost twisted-pair wire and as little expensive coaxial cable as possible between elements of a computer system. A type of twisted pair wire having a line impedance similar to that of coaxial cable can be used. This eliminates the need for impedance matching in a balun, but the need for a balun which will transmit a large bandwidth of frequencies while filtering out unwanted common mode noise remains. Efforts to supply a balun meeting the above requirements have not been successful to date.